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Thursday, December 22, 2016

Ceratosaurus Part 2: What do we actually know about Ceratosaurus teeth?

What you'll notice about both of these diagnoses, however, is that they contain little in the way of dental characters. Sure they both say the premaxilla only contains three teeth, but that is of little help if you are trying to ID isolated teeth. This brings me to the crux of this post in the first place

As I said in a previous post, "Teeth attributed to Ceratosaurus do turn up in the field, however, and are usually distinguished by the presence of ridges near their bases."

Madsen and Welles (2000) found "longitudinal grooves" in the anterior dentary teeth of Ceratosaurus magnicornis (this is not listed in their diagnosis. Another tooth character is though: "the teeth are longer and stouter."), and also cited "persistent parallel grooves on the medial surfaces of the premaxillary teeth and the anterior three teeth of the dentary," in the etymology for C. dentisulcatus (this morphology is not listed as an autapomorphy in their diagnosis - though "teeth more massive" is). Later in their publication, however, Madsen and Welles (p. 35) do state that, "lateral grooves are diagnostic of the premaxillary and anterior three teeth of the Ceratosaurus dentary," when discussing Ceratosaurus sp. from Tanzania. In the next paragraph they discuss "lingual grooves" as being characteristic of Ceratosaurus. They point out that these grooves are not diagnostic to the species level. There are two problems with this: there is now only one recognized species of Ceratosaurus, and the holotype lacks premaxillary and anterior dentary teeth.

While Gilmore does use some dental-based characters, they are not tied to a specific tooth morphology and instead deal with the number of teeth in the various tooth-bearing bones.

So what do we actually know about Ceratosaurus teeth?

Plate 17, Figure 1 from Gilmore (1920), showing the right side of the holotype of Ceratosaurus

Plate 17, Figure 2 from Gilmore (1920), showing the left side of the holotype of Ceratosaurus

The holotype lacks the apparently diagnostic teeth of Madsen and Welles' C. dentisulcatus. As you can see from Gilmore, the left hand side of the skull features no anterior teeth. The right side is equally bleak when it comes to anterior dentition. So we are now faced with a tooth morphology that may be diagnostic (but not used in a formal diagnosis) of a junior synonym that cannot be directly be compared with the holotype.

Continuing to muddy (or clarify?) the waters is C. ?meriani, from the Late Jurassic of Switzerland. Madsen and Welles (2000) figure it and refer it to Ceratosaurus sp., and Mickey Mortimer over at the Theropod Database says, "It differs from Genyodectes and Ostafrikosaurus in lacking mesial serrations. As it is of identical size and found in temporally equivalent beds, I believe it should be called Ceratosaurus meriani." Well there are other teeth from the Late Jurassic that have fluting and lack mesial serrations. Specifically some teeth referred to Ceratosaurus from the Morrison Formation of the American West. But why are they referred to Ceratosaurus? It is the right size and shape and comes from the right aged beds...but that's usually not good enough to assign to a highly exclusive clade.So let's sum up what we can say about the teeth of Ceratosaurus (and please chime in if I've goofed somewhere):

Ceratosaurus nasicornis has no known premaxillary or anterior dentary teeth

C. magnicornis has both premaxillaries preserved but, again, no teeth preserved in situ. It also lacks a dentary

C. dentisulcatus preserves both an toothed premaxilla and dentary with the lingual surfaces of the premaxillary and anterior dentary teeth preserving apicobasal grooves.

Several teeth from the Mygatt-Moore Quarry have been referred to Ceratosaurus on the basis of apicobasal fluting.

So it seems that people are treating these apicobasal flutes (and in some cases the lack of mesial serrations) as diagnostic to Ceratosaurus. But apicobasal flutes are known in more taxa than just Ceratosaurus; several Triassic archosauriform tooth morphotypes possess them, as do temnospondyls, some phytosaurs, some early Jurassic dinosaurs (WFtP), spinosaurs, crocs, and gators. That's just tetrapods; I haven't wanted to dive deep into what sort of crazy dentition fish might have. As I understand it (though I don't have any sources right next to me at the moment) this adaptation would be useful for resisting strain on the tooth; basically the animal was punching through tough, wriggling stuff with its face. Understandable then that many things that eat tough prey would have this sort of adaptation.

Where does that leave us with Ceratosaurus? Well it leaves us with the possibility (not the certainty) that Ceratosaurus had apicobasal fluting on its anterior teeth. Certainly specimens with non-dental remains seem to suggest at least one species referred to Ceratosaurus previously had these flutes. I would urge caution, however, in assigning all Late Jurassic stout, fluted theropod teeth to Ceratosaurus. First off, the holotype doesn't have them, so the referral is based on previously referred specimens. That sort of secondary referral is done relatively frequently but it should give one cause for consideration. The second point is that we really don't have a good idea of the dental variation in other Late Jurassic theropods. We know the teeth of Allosaurus fairly well, but what can we say of the tooth structure at all major tooth positions in Torvosaurus, for example (especially considering derived megalosaurs, the spinosaurs, developed fluted tooth crowns as well)? Do we really know what the teeth were like in some of the mid-sized Morrison taxa as well? Since most aren't represented by good cranial remains we don't know what tooth variation is out there.

In closing, we can say that it is likely Ceratosaurus had fluted front teeth but because of problems with the type specimen and the plesiomorphic nature of dental fluting, the rampant homoplasy in tetrapod dentition in general, and the lack of information about other Morrison theropods' teeth it is unwise to assign isolated fluted teeth to Ceratosaurus at all. Hopefully descriptions of other Ceratosaurus specimens with preserved anterior teeth along with a better understanding of Morrison theropod tooth diversity will lead to more accurate diagnoses of these fluted theropod teeth.

Minor aside

Life happens. I've been off the blog game for almost the whole year but I am hoping to be back at it through 2017, especially in terms of blogging about our next publications we have coming out. I've also brought a couple amigos on board here at the Prehistoric Pub so we plan on at least not letting you all down too much this next year. Onward to the next paleo discovery!

4 comments:

I have two issues with your arguments. The first is that I don't see why basing diagnostic characters on referred specimens "should give one cause for consideration" if you can refer the specimens to the same taxon based on other apomorphies. Ceratosaurus DID have fluting on anterior teeth as shown by the dentisulcatus specimen. Maybe not all individuals did, but if we held to this level of uncertainty for all Mesozoic dinosaurs (most of which are only known from one specimen), our diagnoses wouldn't get far at all.

The second issue is you give consistently present caveats far too much consideration. Most any character will be convergently developed by some other tetrapod, but that doesn't make them less valuable as part of a diagnosis. And when assigning a specimen to a taxon, you don't do it in a vacuum, so that e.g. phytosaurs' fluted teeth aren't a consideration at all because they were extinct by the Late Jurassic. Until you find a fluted example that matches Ceratosaurus' age, size and other anatomical characters, but is not Ceratosaurus, then it's perfectly fine to refer isolated Morrison fluted teeth to the genus. Similarly, the fact we don't know if every other coexisting taxon lacked a character doesn't mean we can't refer based on that character. It would be different if a relative of a Morrison taxon had the character, and the state was unknown for the Morrison taxon. Then you might say that Morrison taxon could also be a contender for owning the isolated fluted teeth, but that's not the case for Ceratosaurus' contemporaries AFAIK. Again, if we held this standard to every taxon, we wouldn't get far at all with diagnoses. This is especially true when we consider basically all Mesozoic ecosystems are only known by a fraction of their probable actual diversity, so that basically every Mesozoic dinosaur had contemporaries whose anatomy is completely unknown and so could have technically had any character state. But that would be a ridiculous reason to not refer material.

In short, vert paleo is a provisional field, where it's perfectly fine to say Ceratosaurus is diagnosed in part by mesially fluted anterior teeth, without having to consider the caveats that are true of any character that maybe future finds of individuals lacking the character or of non-Ceratosaurus taxa with the character will falsify this, or that other distantly related taxa also have the character. Those are just givens.

Thanks for taking the time to read through things. I definitely value your input and appreciate your perspective here.

I wasn't bringing up fluted teeth in phytosaurs to suggest that perhaps some of these Morrison fluted teeth were bizzaro Late Jurassic phytosaurs, clearly. The point is that this is an easy-to-evolve adaptation that has appeared in multiple lineages throughout the last 220 million years, including in unrelated but coeval taxa (such as phytosaurs and metoposaurs). I think it is kind of circular to say that "These fluted teeth are from the Late Jurassic so they must be Ceratosaurus because it is the only Late Jurassic animal with fluted teeth," even when we don't know that to be true. It becomes self-reinforcing, doesn't it?And before you point it out, that doesn't take into account your other point; we don't know the variation in other taxa (some very poorly known) but that it shouldn't prevent us from making assignments, "if we held this standard to every taxon, we wouldn't get far at all with diagnoses."I guess my concern boils down to two things: 1) This isn't actually a character that has been formally diagnosed for Ceratosaurus itself, just something that we all have sort of agreed on but haven't actually tested. 2) Confidently assigning specimens to taxa (and erecting new taxa) because of untested (but assumed) characters in teeth created a lot of the taxonomic mess in Triassic dental studies. Hopefully we can avoid that sort of mistake in the Morrison. I'm not saying that the situations are identical but it could be that we are misinterpreting the data and should be aware/cautious of that going forward.

A secondary point; you said, "It would be different if a relative of a Morrison taxon had the character, and the state was unknown for the Morrison taxon. Then you might say that Morrison taxon could also be a contender for owning the isolated fluted teeth, but that's not the case for Ceratosaurus' contemporaries AFAIK."While not terribly close relatives, spinosaurids are megalosaurs and the premaxillary teeth of Torvosaurus are unknown.

It's not circular to argue that given 1. the only LJ taxon that's been shown to have fluted anterior teeth is Ceratosaurus, and 2. I found isolated fluted anterior teeth from the LJ, thus 3. these are most likely Ceratosaurus teeth. It would only be circular if we defined Ceratosaurus as the animal that all fluted anterior LJ teeth belonged to, so that if we found a Morrison croc with those teeth, we concluded the croc must be Ceratosaurus. Similarly, it would only be self-reinforcing if we counted every isolated tooth thus referred to Ceratosaurus as further confirming evidence all such teeth belonged to Ceratosaurus. But I'm not arguing in that way. I think the fluted dentisulcatus specimen added to the lack of fluted examples in other Morrison taxa or their relatives is good enough for our provisional need.

I don't think being part of a formal diagnosis matters, since these vary highly in quality and are often nullified by close relatives that weren't considered. How many formal diagnoses of Ceratosaurus took into account Genyodectes, Ostafrikaasaurus and Eoabelisaurus (an abelisaurid in some of Wang et al.'s 2016 trees; did you know it had osteoderms too?)?

You claim characters used in diagnoses are bad when they are "untested" and "easy-to-evolve", but what do either of these really mean, and just how many characters used in e.g. Mesozoic theropod diagnoses have been tested or can be evaluated for ease of evolution? Is the latter just a short-hand for the amount of times they've evolved convergently in archosaurs/reptiles/tetrapods/vertebrates?

The premaxillary teeth of Torvosaurus are unknown, but Duriavenator and Dubreuillosaurus lack fluting in theirs. At least the former is always found to be a megalosaurid, and so closer to Torvosaurus than to spinosaurids. You could correctly argue that megalosauroid topologies aren't strongly supported at the moment and that Duriavenator has basically only been tested in variants of Benson's matrix, but this just goes back to my point about there always being caveats in our field.